1. Field of the Invention
This invention relates to internally manifolded fuel cell stacks, and in particular, a method and process for sealing fully internally manifolded cell stacks with wet seals between only the electrolyte and metallic separator plates to provide long term stability.
Generally, fuel cell electrical output units are comprised of a stacked multiplicity of individual cells separated by inert or bi-polar electronically conductive ferrous metal separator plates. Individual cells are sandwiched together and secured into a single stacked unit to achieve desired fuel cell energy output. Each individual cell generally includes an anode and cathode electrode, a common electrolyte tile, and a fuel and oxidant gas source. Both fuel and oxidant gases are introduced through manifolds to their respective reactant chambers between the separator plate and the electrolyte tile. The area of contact between the electrolyte and other cell components to maintain separation of the fuel and oxidant gases and prevent and/or minimize gas leakage is known as the wet seal. A major factor attributing to premature fuel cell failure is corrosion and fatigue in the wet seal area. This failure is hastened by corrosive electrolyte contact at high temperatures and high thermal stresses resulting from large temperature variations during thermal cycling of the cell causing weakening of the structure through intracrystalline and transcrystalline cracking. Such failures permit undesired fuel and/or oxidant gas crossover and overboard gas leakage which interrupts the intended oxidation and reduction reactions thereby causing breakdown and eventual stoppage of cell current generation. Under fuel cell operating conditions, in the range of about 500.degree. to 700.degree. C., molten carbonate electrolytes are very corrosive to ferrous metals which, due to their strength, are required for fuel cell housings and separator plates. The high temperature operation of stacks of molten carbonate fuel cells increases both the corrosion and thermal stress problems in the wet seal area, especially when the thermal coefficients of expansion of adjacent materials are different.
This invention provides fully internal manifolding of the fuel and oxidant gases to the individual cells of an assembled stack in a manner utilizing electrolyte/metal wet seals which, due to the design of the cell components, provides long term endurance and stability of fuel cell operation.
2. Description of the Prior Art
Commercially viable molten carbonate fuel cell stacks may contain up to about 600 individual cells each having a planar area in the order of eight square feet. In stacking such individual cells, separator plates separate the individual cells with fuel and oxidant each being introduced between a set of separator plates, the fuel being introduced between one face of a separator plate and the anode side of an electrolyte matrix and oxidant being introduced between the other face of the separator plate and the cathode side of a second electrolyte matrix.
The emphasis in fuel cell development has been in external manifolding of the fuel and oxidant gases by using channel manifolds physically separable from the fuel cell stack. However, the inlets and outlets of each cell must be open to the respective inlet and outlet manifolds which must be clamped onto the exterior of the cell stack. To prevent electrical shorting, insulation must be used between the metal manifolds and the cell stack. External manifolding has presented serious problems in maintaining adequate gas seals at the manifold/manifold gasket/cell stack interface while preventing carbonate pumping within the gasket along the potential gradient of the cell stack. Various combinations of insulating the metal manifold from the cell stack have been used, but with the difficulty of providing a sliding seal which is gas tight and electrically insulating while being carbonate impermeable under high temperature molten carbonate fuel cell operating conditions, no satisfactory solution has been found. The problem of manifolding and sealing becomes more severe when larger number of cells and larger planar areas are used in the cell stack. When greater number of cells are used, the electrical potential driving the carbonate in the seal area along the height of the stack increases, and when the planar area of the cell increases, the linear tolerances of each component and the side alignment of each component becomes extremely difficult to maintain in order to maintain the mating surface sealed between the manifold/manifold gasket/and cell stack.
Cell stacks containing 600 cells can be approximately 10 feet tall presenting serious problems of required stiffness of external manifolds and the application of a clamping force required to force the manifold onto the cell stack. Due to the thermal gradients between cell assembly and cell operating conditions, differential thermal expansions, and the necessary strength of materials used for the manifolds, close tolerances and very difficult engineering problems are presented.
Conventionally, stacks of individual molten carbonate fuel cells have been constructed with spacer strips around the periphery of a separator plate to form wet seals and to provide intake and exhaust manifolds. Various means of sealing in the environment of the high temperature fuel cell wet seal area are disclosed in U.S. Pat. No. 4,579,788 teaching the wet seal strips are fabricated utilizing powder metallurgy techniques; U.S. Pat. No. 3,723,186 teaching the electrolyte itself is comprised of inert materials in regions around its periphery to establish an inert peripheral seal between the electrolyte and frame or housing; U.S. Pat. No. 4,160,067 teaching deposition of inert materials onto or impregnated into the fuel cell housing or separator in wet seal areas; U.S. Pat. No. 3,867,206 teaching a wet seal between electrolyte-saturated electrodes; U.S. Pat. No. 4,761,348 teaching peripheral rails of gas impermeable material to provide a gas sealing function to isolate the anode and cathode from the oxidant and fuel gases, respectively; U.S. Pat. No. 4,329,403 teaching graded electrolyte composition for more gradual transition in the coefficient of thermal expansion in passing from the electrodes to the inner electrolyte region; and U.S. Pat. No. 3,514,333 teaching housing of alkali metal carbonate electrolytes in high temperature fuel cells by use of a thin aluminum sealing gasket. None of the above patents deal with sealing around internal fuel and oxidant in fuel cell stacks.
Gas sealing of a phosphoric acid fuel cell, which operates at about 150.degree. to 220.degree. C., by filling the pores of a porous material periphery of the cell constituents with silicon carbide and/or silicon nitride is taught by U.S. Pat. No. 4,781,727; and by impregnating interstitial spaces in substrate plate edge is taught by U.S. Pat. Nos. 4,786,568 and 4,824,739. The solution of sealing and corrosion problems encountered in low temperature electrolytic cells, such as bonding granular inert material with polytetrafluorethylene as taught by U.S. Pat. No. 4,259,389 gaskets of polyethylene as taught by U.S. Pat. No. 3,012,086; and "O" ring seals taught by U.S. Pat. No. 3,589,941 for internal manifolding of fuel only are not suitable for high temperature molten carbonate fuel cells.
U.S. Pat. No. 4,510,213 teaches transition frames surrounding the active portion of the cell units to provide fuel and oxidant manifolds to the gas compartments of the individual cells, the manifolds not passing through the separators nor the electrolyte tiles of the cells. The transition frames require complicated insulating between adjacent cells and are made up of several separate and complicated components. U.S. Pat. No. 4,708,916 teaches internal manifolding of fuel and external manifolding of oxidant of molten carbonate fuel cells wherein sets of fuel manifolds pass through electrodes as well as electrolytes and separators in a central portion and at opposite ends of the individual cells to provide shortened fuel flow paths. The end fuel manifolds are in a thickened edge wall area of the separator plate while the central fuel manifolds pass through a thickened central region and sealing tape impregnated with carbonate or separate cylindrical conduit inserts are provided extending through the cathode.
Internal manifolding has been attempted wherein multiple manifold holes along opposite edges of the cell have been used to provide either co- or counter-current flow of fuel and oxidant gases. These manifold holes for fuel have been located in a broadened peripheral wet seal area along opposing edges, but the manifolds have been complicated structures exterior to the electrolyte or pass through at least one of the electrodes. However, adjacent manifold holes are used for fuel and oxidant which provides short paths across a short wet seal area and leakage of the gases as well as the necessarily broadened peripheral seal area undesirably reducing the cell active area. Likewise, prior attempts to provide internal manifolding have used multiple manifold holes along broadened peripheral wet seal areas on each of all four edges of the cell to provide cross flow, but again short paths between adjacent fuel and oxidant manifold similar complicated structures and holes caused leakage of the gases and further reduced the cell active area.